The present invention relates to a compression piston ring for use in an internal combustion engine, and more particularly, to a type thereof capable of providing a superior advantage when used as a first compression piston ring of a compression-ignition engine (diesel engine), the first compression piston ring being positioned closer to a combustion chamber than is a second and subsequent compression ring(s).
Conventionally, a low point piston ring has been widely used in which an ovality of the ring is set not more than zero to provide an elliptic shape so as to reduce the surface pressure at the position nearby gap of the ring at a room temperature, thereby restraining increase in the surface pressure at the gap end during actual engine operation, to thus avoid generation of cracks and peel-off of a surface treatment layer of the ring.
Japanese Patent Application Publication No. 2000-120866 discloses a compression ring in which after the ring is subjected to surface treatment, a notched portion is formed at an inner peripheral surface of the ring at a position near the gap so as to partly decrease a radial length of the ring (the radial length being corresponding to xe2x80x9ca1xe2x80x9d in FIG. 7). This is an improvement on conflicting problems in that sufficient wear resistance is not obtained if attention is only drawn to the prevention of partial gap between an outer peripheral surface of the piston ring and the inner peripheral surface of the cylinder liner, and the partial gap may occur if attention is only drawn to the reduction in surface pressure at the gap. According to the disclosed piston ring, increase in surface pressure at the gap during engine operation can be restrained by a change in a ring curvature and by the reduction in bending rigidity near the gap, those being caused by the formation of the notch. Accordingly, generation of crack and peel-off of the surface treatment layer can be avoided. Such piston ring can be used in a supercharging type diesel engine providing a severe operating condition.
In a low-point piston ring whose ring shape is regulated and in the piston ring disclosed in Japanese Patent Application Publication No. 2000-120866, surface pressure increase near the gap can be restrained to reduce the excessive wear of the sliding surface.
However, in such a low-point piston ring produced by regulating the ring shape, surface pressure near the gap is lower than that of a piston ring providing an equal surface pressure over its periphery at a room temperature. Accordingly, a problem occurs in machining the outer peripheral surface of the low-point ring, particularly in a lapping process. More specifically, generally, during machining to the outer peripheral surface of the ring, a grinding jig is in pressure contact with the outer peripheral surface with a constant pressure at any portion of the ring in a circumferential direction thereof because of the self expanding property of the ring so as to provide an ultimate configuration of the sliding surface of the ring. However, in case of the low-point piston ring wherein the circumferential shape is regulated, the surface pressure near the gap is provisionally lowered prior to the machining to the outer peripheral surface. Therefore, contacting degree between the grinding jig and the ring is lowered, thereby disabling desirable machining for providing a desired shape near the gap.
In order to obviate this problem, a proposal has been made in that the ring and the grinding jig are provisionally heated to provide a temperature distribution similar to that in the engine operation in an attempt to allow the ring portion near the gap to be in intimate contact with the grinding jig because of thermal deformation. However, in reality, it would be extremely difficult to perform machining with the simulated temperature distribution.
Further, in the piston ring formed with the notch at the inner peripheral surface and near the gap as disclosed in Japanese Patent Application Publication No. 2000-120866, the radial length (corresponding to xe2x80x9ca1xe2x80x9d in FIG. 7) is locally reduced by locally cutting and grinding the inner peripheral surface of the ring near the gap after surface treatment to the ring. If an entire surface of the ring is subjected to nitriding treatment, cutting efficiency is extremely lowered due to high hardness of the nitriding layer. Accordingly, shortened service life of a cutting tool results. Even if the surface treatment to the ring has not been made, great amount of the ring part must be removed by cutting and grinding for the formation of the notch. This requires a prolonged machining period. Consequently, production cost is greatly increased with the addition of the process of machining and grinding to nearby the gap.
It is an object of the present invention to overcome the above-described problems and to provide a compression piston ring for an internal combustion engine capable of restraining excessive surface pressure increase nearby the gap during engine operation and capable of being produced at a low cost.
This and other objects of the present invention will be attained by a compression piston ring for use in an internal combustion engine, the piston ring having a generally circular shape and having an outer peripheral sliding surface in sliding contact with a cylinder and an inner peripheral surface in confrontation with a piston, a gap being formed to provide a discontinuity of a circle. The compression piston ring proves d1/a1 ranging from 40 to 60 and provides d1/h1 ranging from 30 to 55, in which d1 represents an inner diameter defined by the inner peripheral surface, a1 represents a radial distance between the inner peripheral surface and the outer peripheral sliding surface, and h1 represents an axial length of the outer peripheral sliding surface.
In another aspect of the invention, there is provided a diesel engine including a cylinder, a piston reciprocally movable with respect to the cylinder, and having an outer peripheral surface formed with a plurality of annular piston ring grooves, a combustion chamber being defined above the piston, and the above described first compression piston ring assembled in a first piston ring groove positioned closest to the combustion chamber among the plurality of the piston ring grooves.
The above described range of d1/a1 is relatively greater than that of the conventional compression piston ring, and a1 dimension at not only the gap and near the gap but also at overall circumference of the piston ring of the present invention is relatively smaller than that of the conventional piston ring. With this arrangement, a distance between the inner and outer peripheral surfaces of the piston ring is reduced to provide a moderate temperature gradient therebetween. Accordingly, excessive increase in surface pressure at the gap and a portion adjacent thereto can be suppressed. Further, sealing performance between the upper end face of the piston ring and the upper surface of the piston ring groove and between the lower end face of the piston ring and the lower surface of the piston ring groove can be maintained, and mobility of the piston ring within the piston ring groove can be stabilized during engine operation.
Further, because a1 dimension is constant over the entire circumference of the piston ring, machining to a particular inner peripheral surface portion of the piston ring is not required, thereby lowering production cost. Furthermore, because d1/h1 ranges from 30 to 55, wear at the upper and lower surfaces of the piston ring groove can be restrained thereby maintaining sealability between the piston ring and the piston ring groove at the upper and lower surfaces. Moreover, this dimensional limitation can maintain heat transmitting efficiency of the piston ring for transmitting heat from the piston to the cylinder and can also maintain self expansion force of the piston ring.
If the compression piston ring of the present invention is used in a supercharging type diesel engine, the piston ring is exposed to extremely high engine pressure during engine operation. Even though the compression piston ring does not provide high self-expansion force because of small a1 dimension, the small a1 dimension can provide low bending rigidity of the piston ring, so that the outer peripheral sliding surface of the piston ring can be brought into intimate contact with the cylinder during engine operation by the high engine pressure applied to the inner peripheral surface of the piston ring. Accordingly, a combustion gas and lubrication oil can be effectively sealed. If the compression piston ring is used as a first compression piston ring to which the engine pressure is directly applied, the sufficient sealing function results.